For industrial and utility applications, the system is generally expected to
survive for 20–40 years under various operating stresses.
Novel insulation materials and construction are always under development, but certain material combinations are known to perform consistently well.
Modern insulation systems usually feature a combination of organic and
inorganic materials, such as slit tapes of mica-flake paper
combined with glass fibers and/or polymeric materials
that are wrapped around copper conductors (“strands”).
These are covered with combinations of polymer, glass,
or mica flake. The resulting composite is VPI with a specially
formulated epoxy or polyesterimide resin.
Some designs may use resin-rich systems, whereby the insulation
is press-cured before it is inserted in the stator slots.
In very old machines, one may find systems constructed
of natural materials, but in the past 50 years or so, the
development of synthetics with improved thermal capability
and ease of handling has increased the expectation
of an insulation system to meet its application demands
with far less material in construction [1], [2]